The purpose of radiation therapy is tumor control. In view of clonogen proliferation it makes sense to deliver the necessary dose in as short a time as possible. The limiting factor in tumor treatment is normal tissue reaction: normal tissue reactions must not exceed "tolerance" level. The development and application of high technology particularly in computers and computer-based and assisted imaging has stimulated great progress in tumor localization and treatment planning, and even in the technology of delivery and its quality assurance, i.e., the spatial aspects of the issue. Decision-making as to the amount of dose and its distribution over time (by fractionation or protraction) is still essentially empirical, however. The present project continues the development and exploration of a theoretical description of time-dose response of tumors as well as normal tissues. Its basic concepts have been published in 1988[1]. Since then, further developments have concentrated on extension of the Linear-Quadratic model. The extension concerns a unified description of the influence of incomplete repair and comprises a description of the influence of tumor proliferation and stem cell/transition cell repopulation. A study regarding the implications of high dose arate vs. low dose rate brachytherapy is ongoing. This work concentrates on the implications of uncertainties in the relevant parameters, particularly a/b and the time factor. The work includes the development of an interactive computer program and graphical tools to help clinical guidance in the search for high dose rates compatible with constant tumor control as well as no more severe toxicity.